The VectorRenderer draw arrows from x,y to x+dx, y+dy. The major issue that I see is that the renderer does the conversion from data space (the dx and dy values) to Java 2D space (the length with which the lines are drawn in the chart) by the domain and range axis. Since the axes represent time, and the length of the arrow should represent the magnitude, this will cause problems.
You might be better of by a) using a WindDataset and a WindItemRenderer or by b) subclassing VectorRenderer, overwrite drawItem, and calculate xx1 and yy1 by driect conversion of the magnitude to a length in Java2D coordinates.